Silicon crystals are produced by being pulled and grown using the Czochralski (CZ) method. The grown silicon crystal ingot is then sliced into silicon wafers. Semiconductor devices are fabricated by operations in which device layers are formed on the surface of the silicon wafer.
However, in the course of silicon crystal growth, crystal defects known as “grown-in defects” arise.
With the progress in recent years toward higher density and smaller geometries in semiconductor circuits, the presence of such grown-in defects near the surface layer of the silicon wafer where devices are fabricated can no longer be tolerated. This situation has led to explorations on the possibility of manufacturing defect-free crystals. The following three types of crystal defects are detrimental to device characteristics:    a) Crystal originated particles (COP) are void defects which arise from the coalescing of vacancies;    b) Oxidation-induced stacking faults (OSF); and    c) Dislocation loop clusters that arise from the aggregation of interstitial silicon (also known as interstitial silicon dislocation defects, or I-defects).
A defect-free silicon monocrystal is recognized or defined as a crystal which is free or substantially free of the three above types of defects.
One method for obtaining silicon wafers that are free of grown-in defects near the surface layer where device circuits are created is to use epitaxial growth to grow a defect-free layer on the wafer surface.
That is, an epitaxial silicon wafer is a high value-added silicon wafer carefully created by the vapor-phase growth of an epitaxial growth layer (also known as an “epilayer”) having a high degree of crystal perfection on a silicon wafer substrate. Because the epitaxial growth layer has a high degree of crystal perfection, it is thought to be a substantially defect-free layer. Hence, device fabrication on an epitaxial growth layer dramatically enhances the device characteristics compared with device fabrication on the surface layer of a silicon wafer substrate. Because the degree of crystal perfection in the epitaxial growth layer was not thought to be strongly affected by the crystal qualities of the underlying silicon wafer substrate, little importance has been placed until now on the quality of the silicon wafer substrate itself.
Prior-Art 1
However, in recent years, as the systems used to inspect defects have become increasingly sensitive and the criteria for evaluating defects have become more exacting, it has been found that defects within the silicon wafer substrate propagate to the epitaxial growth layer, where they appear as defects in the epitaxial growth layer (referred to herein as “epitaxial defects”). This is described in Non-Patent Reference 1 (Sato: 16th Meeting of Silicon Technology Division, Japan Society of Applied Physics; Apr. 24, 2000; p. 35).
Device manufacturers have thus begun calling for the production of epitaxial defect-free epitaxial silicon wafers having an epitaxial growth layer that is free of defects by forming an epitaxial growth layer on a silicon wafer substrate which is free of crystal defects that cause epitaxial defects.
Grown-in defects in a silicon wafer substrate include defects which readily propagate to the epitaxial growth layer and defects which do not readily propagate. OSFs and dislocation loop clusters in particular are very likely to propagate to the epitaxial growth layer and become epitaxial defects, and so must be excluded from the silicon wafer substrate.
If the temperature gradient G in the crystal axis (vertical) direction is assumed to be constant, the defects in a silicon monocrystal vary with the pull rate V of the silicon monocrystal. In other words, it is known that, as the pull rate V decreases from a high speed, void defects (COPs), OSFs (ring-like OSFs, abbreviated as “Ring-OSF,” which are stacking faults observed on a ring concentric with the center of the wafer following heat treatment in an oxidizing atmosphere), defect-free regions and dislocation loop clusters arise one after another.
In P-type silicon crystals, boron (B) is added to the silicon crystal as a dopant. In p/p+ and p/p++ epitaxial silicon wafers containing a high concentration of boron, about 1×1018 atoms/cm3 to 1×1019 atoms/cm3 of boron have been added to the silicon crystal.
Prior-Art 2
Non-Patent Reference 2 (E. Dornberger, E. Graff, D. Suhren, M. Lambert, U. Wagner and W. von Ammon: Journal of Crystal Growth, 180 (1997), 343) describes the influence of boron on the behavior of crystal defects. This Non-Patent Reference 2 discloses that adding boron to a high concentration in a silicon crystal results in the generation of R-OSFs at a higher pull rate V.
Conditions currently used for the production of p+ and p++ silicon crystals are described here with referring to attached diagrams according to the present invention.
FIG. 2A shows the distribution of epitaxial defect regions and epitaxial defect-free regions. The vertical axis represents the normalized pull rate V/Vcri when the temperature gradient G in the crystal axis (vertical) direction is assumed to be constant, and the horizontal axis represents the concentration of boron added to the silicon crystal in atoms/cm3. The normalized pull rate V/Vcri refers herein to the pull rate normalized by the critical rate Vcri when the added boron concentration is 1×1017 atoms/cm3, and the critical rate Vcri refers to the pull rate at which R-OSFs at the center of the silicon crystal disappear when the pull rate V is gradually decreased.
Epitaxial defect-free region α1 in FIG. 2A is an epitaxial defect-free region where void defects appear in the silicon wafer substrate and the epitaxial growth layer is free of defects. Epitaxial defect region β1 is an epitaxial defect region where OSFs appear in the silicon wafer substrate and defects appear in the epitaxial growth layer. Epitaxial defect-free region α2 is an epitaxial defect-free region where the silicon wafer substrate is free of defects and the epitaxial growth layer is free of defects. Epitaxial defect region β2 is an epitaxial defect region where dislocation loop clusters appear in the silicon wafer substrate and defects appear in the epitaxial growth layer.
Up until now, p+ silicon crystals have been produced in the region indicated as J (called the production conditions region) in FIG. 2A. The production conditions region J includes epitaxial defect region β1. In order to suppress epitaxial defects, attempts have been made to produce silicon crystals within epitaxial defect-free region α2 by moving the production conditions region to a lower V side—that is, to the production conditions region K shown in FIG. 2B.
Prior-Art 3
Here, in low boron concentration p− silicon crystals (having a boron concentration of less than 1×1018 atoms/cm3), when the pull rate V is lowered, defects arise in the epitaxial growth layer due to dislocation loop clusters. However, in high boron concentration p+ and p++ silicon crystals, Non-Patent Reference 3 (Asayama, et al.: 1999 Fall Meeting of Japan Society of Applied Physics; 3p-ZY-4) reports the suppression of dislocation loop clusters even at the same low pull rate V.
Accordingly, it was previously thought that, in the production of high-boron concentration p+ and p++ silicon crystals, lowering the pull rate V would enable the relatively easy production of high-quality silicon crystals without epitaxial defects. That is, it was predicted that a lower limit for the epitaxial defect-free region α2 exists at low boron concentrations (less than 1×1018 atoms/cm3), but does not exist at high boron concentrations (1×1018 atoms/cm3 to 1×1019 atoms/cm3).